Programmable infusion pumps for delivering nutritional liquids and medicine to patients in accordance with predetermined liquid delivery parameters are in wide usage. One type of infusion pump is a peristaltic pump arranged along flexible connective tubing carrying liquid from a liquid source to the patient. The peristaltic pump has a pumping mechanism for progressively squeezing successive portions of the tubing to cause fluid to flow through the tubing in a flow direction toward the patient. In a common arrangement, the pumping mechanism includes a motor-driven wheel having radial fingers or rollers that engage a segment of the tubing arranged about a circumferential portion of the wheel. As the wheel rotates, fluid is pumped through the tubing to the patient. The tubing segment arranged about the pump wheel may be held in a U-shaped configuration by a cassette designed for receipt in a channel or receptacle area of the pump. The cassette may provide terminals for connecting an incoming line of tubing coming from the liquid source and an outgoing line of tubing going to the patient to opposite ends of the U-shaped tubing segment received by the pump.
A recognized safety concern when pumping nutritional liquids for enteral feeding or medicinal fluids for intravenous therapy is the formation of air bubbles in the liquid being pumped into the patient. As a safety measure, it is known to provide an air-in-line sensor on the infusion pump for detecting an air-in-line condition and triggering an alarm. For example, the air-in-line sensor may include an ultrasonic transmitter arranged to direct ultrasound through the tubing and a receiver on an opposite side of the tubing from the transmitter for receiving the ultrasound waves after passage through the tubing and the fluid carried thereby. The receiver generates an output signal indicating whether the ultrasound signal passed through liquid or air as it travelled from the transmitter to the receiver.
The air-in-line sensor output is sampled regularly as fluid is pumped through the tubing to observe each incremental volume of fluid passing through the sensor's zone of observation. In known air bubble detection algorithms, an air-in-line alarm condition is detected when a series of consecutive sensor readings indicate that a predetermined volume of air has passed the sensor (e.g. 1.5 milliliters) without the presence of a predetermined volume of liquid (e.g. 0.375 milliliters).
A problem has been identified that occurs when food bottles containing a nutritional liquid are vigorously shaken to mix the contents. In such cases, micro-bubbles may collect at the downstream side of the air-in-line sensor and may eventually cause an air-in-line alarm. The delivery of fluid by the pump may be implemented in discrete time segments during which the pump's motor is actually on and pumping only a small portion of the time segment, and is off for the remainder of the time segment. Due to gravity, air micro-bubbles caused by shaking may float upstream and gather at the air-in-line sensor, potentially causing detection of an air-in-line condition which will trigger a “false” alarm.
A need exists to prevent this type of false alarm, preferably without changing the pump hardware or sensor hardware.